Method and apparatus for enhancing the rate and efficiency of gas phase reactions

ABSTRACT

An apparatus and a method for enhancing the rate of a chemical reaction in a gas stream. The apparatus includes at least one heterogeneous catalyst having an upstream end and a downstream end, and at least one surface having a plurality of catalytically active sites on the surface, where the catalyst is positioned so that at least a portion of the gas stream contacts at least a portion of the catalytically active sites on the surface. At least one device for producing radicals or other active species from at least one of water vapor or other gaseous species, such as a corona discharge device or a UV light source is used to produce radicals or other active species, which are introduced into the gas stream at a position upstream of the downstream end of the catalyst. The radicals or other active species are introduced in an amount sufficient to reduce or eliminate poisoning of the catalyst by catalyst poisons, such as sulfur, sulfur containing compounds, phosphorous, phosphorous containing compounds, and carbon.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 09/122,394 Jul. 24, 1998U.S. Pat. No. 6,047,543.

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 08/947,287, filed Oct. 7, 1997 now U.S. Pat. No.6,029,442 which is a continuation-in-part of U.S. patent applicationSer. No. 08/768,833, filed Dec. 18, 1996 now U.S. Pat. No. 5,863,413,the teachings of which are incorporated herein in their entirety byreference.

FIELD OF THE INVENTION

The present invention is directed to a method and apparatus forimproving and maintaining the performance of catalytic reactors andcatalytic convertors, particularly catalytic reactors used in fuel cellsfor producing electricity and vehicle catalytic convertors used toreduce the emission of pollutants. More particularly, the invention isdirected to a method and apparatus where the improved performance of thecatalysts is achieved by producing highly oxidizing free radicals, suchas hydroxyl radicals, OH, hydroperoxyl radicals, HO₂, atomic hydrogen,H, and atomic oxygen, O, and other active species, including relatedoxidizing gaseous species, such as hydrogen peroxide, H₂O₂, nitrogendioxide, NO₂, and ozone, O₃, by any means known in the art, butpreferably with a corona discharge, and introducing these active speciesinto the gas stream flowing into and through a catalyst, such as thecatalytic reactor fuel reformer used to produce hydrogen gas from ahydrocarbon fuel for use in a fuel cell, a catalytic combuster, or acatalytic convertor associated with an internal combustion engine.

BACKGROUND OF THE INVENTION

Heterogeneous catalysts have been shown to be useful in enhancing therate and/or efficiency of gas phase reactions in a number ofapplications. These applications include emerging technologies, such ascatalytic reactors or fuel reformers that are used to produce hydrogengas, H₂, from hydrocarbon fuels, such as gasoline, natural gas, andalcohols, as well as relatively mature technologies, such as thecatalytic convertors used to reduce the emission of pollutants fromautomobile and truck engines. The performance of heterogeneous catalystsmay be severely degraded by exposure to catalyst poisons, such as thesulfur and phosphorous compounds that are found in varying amounts inautomotive fuels, such as gasoline. As gasoline is expected to be used,at least initially, in automotive applications of fuel cells, thepossible poisoning of both fuel cell catalytic reactors, automotivecatalytic convertors, and other catalytic combusters by fuelcontaminants is a major concern regarding the effectiveness of thesedevices.

Fuel cells are electrochemical devices that convert the chemical energyof a fuel directly into electrical and thermal energy, and have beenused for a number of years in aerospace applications, such as the spaceshuttle, where hydrogen and oxygen gas are combined to produce electricpower. In a typical fuel cell, a gaseous fuel, e.g., hydrogen, H₂, isfed continuously to an anode or negative electrode compartment, and anoxidant, e.g., oxygen or an oxygen containing gas, which is typicallyair, is fed continuously to a cathode or positive electrode compartment.The hydrogen and oxygen are combined at the electrodes, producing waterand an electric current. In addition to water, fuel cells that utilizecatalytic reactors to produce hydrogen gas from hydrocarbon fuels alsorelease carbon dioxide, and may also release very small amounts ofcarbon monoxide.

Theoretically, a fuel cell is capable of producing electrical energy foras long as the fuel and oxidant are supplied to the electrodes. However,pure hydrogen is difficult to store, particularly in a vehicle, and itsuse may not be practical in many applications. In those cases, acatalytic fuel reformer may be used to produce hydrogen gas from ahydrocarbon fuel, and, thus, the life and performance of the fuel cellis limited by the performance and efficiency of the catalytic reactor.As discussed above, if one or more catalyst poisons are present in thefuel used to produce hydrogen in the catalytic fuel reformer, theperformance of the reformer will be degraded, thereby reducing theperformance of the fuel cell.

In addition, fuel cells are sensitive to carbon monoxide, and, thus, theamount of carbon monoxide is typically minimized in the fuel gas byremoval by the catalytic reactor to achieve optimum efficiency of thefuel cell. However, where the catalyst is contaminated or poisoned,carbon monoxide will remain in the fuel gas after passing through thecatalytic reactor. Therefore, for the fuel cell to function efficiently,the catalyst should be substantially free of poisons that prevent theremoval of carbon monoxide from the fuel gas.

Similarly, in virtually all modern gasoline engines used in vehicles,such as automobiles and light trucks, the exhaust gases produced duringcombustion of fuel are conveyed by an exhaust pipe to a catalyticconverter where pollutants, such as carbon monoxide (CO), hydrocarbons(HC), and oxides of nitrogen (NO_(x)), are substantially converted tonon-polluting species, and, thus, are removed from the exhaust gas. Inaddition, it is expected that catalytic convertors will soon bedeveloped for use with diesel engines. Most modern engines employ threeway catalytic converters (“TWC”), which simultaneously oxidize CO and HCto CO₂ and H₂O, and reduce NO and NO₂ to N₂. The amount of CO, HC,NO_(x), and other pollutants produced will vary with the design andoperating conditions of the engine and the fuel and air used. Inparticular, as with fuel cell catalytic reactors, the presence ofcatalyst poisons in the fuel will result in a degradation of theperformance of the catalytic convertor, and, thus, an increase in theamount of pollutant released into the air.

In general terms, a catalytic convertor used with an internal orexternal combustion engine may be considered to be a sophisticatedcatalytic combuster, which is typically used to enhance the oxidation ofa fuel to produce heat. The heterogeneous catalyst in a catalyticcombuster provides a surface on which a fuel and an oxidizer react. In atypical catalytic combuster, a vaporized fuel and air are passed overthe surface of the catalyst. By providing a catalytic site for thereaction of the fuel and oxidizer, the catalyst lowers the activationenergy of the reaction, allowing the reaction to occur at a lowertemperature with greater efficiency. However, the presence of catalystpoisons that may be adsorbed onto the catalyst surface in any of thefuel, oxidizer, or reaction products will degrade the performance andthe efficiency of the catalytic combuster by occupying active sites onthe catalyst surface. This reduces the number of sites available to thefuel and oxidizer, decreasing the reaction rate.

In general terms, the heterogeneous catalysts, used in fuel cellcatalytic fuel reformers or reactors, vehicle catalytic convertors, andcatalytic combusters, provide a catalytic surface that enhances thereaction rate and efficiency of various gas phase reactions. Although anumber of different heterogeneous catalysts are known, the heterogeneouscatalysts used in catalytic reactors and catalytic convertors usuallyutilize a noble metal catalyst. The structure of the catalyst supportmay vary, depending on the application, e.g., ceramic beads that arecoated with the catalytic material may be used. However, where a largethroughput of gas is required, the noble metal catalyst is preferablyheld in a honeycomb monolithic structure, which has excellent strengthand crack-resistance under physical and thermal shock.

The honeycomb construction and the geometries chosen provide arelatively low pressure drop and a large total surface area thatenhances the mass transfer controlled reactions that produce fuel forthe fuel cell or remove pollutants from the exhaust of an engine. Thehoneycomb is often set in a steel container, and protected fromvibration by a resilient matting where needed. Although a singlecatalyst may be use, a typical modern three way catalytic convertorcomprises an outer steel shell that contains at least two honeycombcatalyst “bricks”, i.e., honeycomb monolithic structures holding thenoble metal catalyst, as described above, where one of the bricks ismounted at the upstream, inlet end of the catalytic convertor, and thesecond is mounted at the downstream, outlet end of the catalyticconvertor.

An adherent washcoat, frequently made of stabilized gamma alumina orcorderite into which the catalytic components are incorporated, isdeposited on the walls of the honeycomb. Modern three way catalyticconverters for simultaneously converting all three pollutants typicallyutilize the precious or noble metals platinum (Pt) and rhodium (Rh),where the Rh is most responsible for the reduction of NO_(x), while alsocontributing to CO oxidation, which is primarily performed by Pt.Recently palladium, Pd, which is less expensive, has been substitutedfor or used in combination with Pt and Rh. The active catalyst generallycomprises about 0.1 to 0.15% of these metals. For other applications,where reduction of NO_(x) is not required, so that only the oxidation ofCO or HC are required, rhodium is typically not present in the catalyst.Instead the catalyst is platinum, palladium, or a combination ofplatinum and palladium.

Because the exhaust gases of the combustion process in most modernautomotive gasoline engines tend to oscillate from slightly rich toslightly lean, an oxygen storage medium is added to the washcoat ofvehicular catalytic convertors to adsorb oxygen onto the surface of thewashcoat during any lean portion of the cycle, and release the oxygenfor reaction with excess CO and HC during any rich portion of the cycle.Cerium Oxide (CeO₂) is frequently used for this purpose due to itsdesirable reduction-oxidation response.

The conversion efficiency of a gas phase reaction heterogeneous catalystis measured by the ratio of the rate of mass conversion or removal of aparticular constituent of interest to the mass flow rate of thatconstituent into the catalytic. The conversion efficiency of a catalystis a function of many parameters including aging, temperature,stoichiometry, the presence of any catalyst poisons, such as lead,sulfur, carbon and phosphorous, the type of catalyst, and the amount oftime the gases reside in or on the catalyst.

As discussed above, catalyst poisons, such as sulfur and phosphorous,degrade the performance of catalysts. The performance of catalyticconvertors, catalytic fuel reformers or reactors, catalytic combustorsof various types, and heterogeneous catalysts in general are affected bysuch poisons. Poisons, even in small concentrations, strongly bond tocatalytic sites on the surface of the catalyst, and block the completionof the chemical processes that the catalyst is intended to promote. Thepoisoning of vehicular catalytic convertors by sulfur in gasoline hasbeen a problem, and is expected to also be a severe problem in fuel cellcatalytic reactors that are proposed for automotive applications, wherethe required hydrogen gas will, in all likelihood, initially be producefrom gasoline.

The issue of catalyst poisoning is not new. For example, theeffectiveness of automotive catalytic convertors is severely degraded bythe presence of lead in gasoline. Therefore, the introduction ofcatalytic convertors on production automobiles in the mid-1970'srequired the elimination of tetra-ethyl lead as an octane enhancer infuels. Although the elimination of the lead based octane enhancerrequired research into alternative octane enhancers, it did not requireany major changes in the manner in which the fuel itself is refined,and, thus, the cost of eliminating tetra-ethyl lead from gasoline wasnot prohibitive. However, the elimination of sulfur, a naturallyoccurring element in crude oil, from fuel may be far more expensive.

Now that lead has been essentially eliminated from motor vehicle fuel inthe United States, sulfur is the key component in gasoline responsiblefor the poisoning of catalysts. Sulfur, typically adsorbed in the formof oxides of sulfur, attaches or binds to catalytically active areas onthe surface of the catalyst, such as those used in catalytic combusters,catalytic convertors, and catalytic fuel reformers or reactors. Theadsorption of at least one of sulfur and sulfur compounds prevents theresulting poisoned areas from participating in the gas phase reaction,such as the oxidation of HC and CO, and the reduction of NO_(x) in anautomotive catalytic convertor, and thereby reduces the efficiency ofthe catalyst. As a result, the emission of pollutants is increased wherethe catalyst is used in an internal combustion engine catalyticconvertor. Similarly, it is expected that the presence of sulfur ingasoline will degrade the performance of catalytic reactors used toproduce hydrogen from gasoline to be used as fuel in a fuel cell.

The sulfur content of gasoline presently varies from state to state andfrom refinery to refinery. Where California has a limit on gasolinesulfur content of approximately 30 parts per million by weight (“ppm”),other states have much higher limits on sulfur, and, as a result, sulfurlevels in fuel can exceed 900 ppm. Therefore, there has been a pushwithin the Environmental Protection Agency (“EPA”) to set a nationalstandard for gasoline sulfur content. However, even at the proposedlevel of 80 ppm, a degradation of the performance and efficiency ofcatalytic convertors and catalytic reactors using a fuel containing thatlevel of sulfur is expected.

Alternative methods for reducing sulfur poisoning of heterogeneouscatalysts are available. For example, the catalyst may be heated to atemperature significantly higher than the normal operating temperatureto decompose and/or drive off certain poisons, and thereby recover thepoisoned catalyst. However, the high temperature required cansignificantly reduce the life expectancy of a catalytic device, and isfrequently not possible during normal operation.

Attempts to remove the sulfur compounds that poison catalysts from thegas or exhaust stream before poisoning of the catalyst occurs by directfiltering or by oxidation of SO₂ to SO₃, either catalytically in thepresence of oxygen, i.e., lean conditions, or in a plasma discharge,have been largely unsuccessful. While each of these methods has beenexplored for automotive applications, they often fail to remove anysignificant amount of sulfur or oxides of sulfur, and requiresignificant amount of power. Moreover, these methods may not be feasiblewith fuel cell catalytic fuel reformers at all.

Therefore, a need exists for a simple, inexpensive means of maintainingthe efficiency of gas phase heterogeneous catalysts, such as those usedin automotive catalytic convertors, fuel cell catalytic reactors, andcatalytic combusters. The present invention provides such a means.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus for enhancing the rateof a chemical reaction in a gas stream. The apparatus comprises at leastone heterogeneous catalyst having an upstream end, a downstream end, andat least one surface having a plurality of catalytically active sites onthe surface, and at least one device for producing radicals or otheractive species from water vapor and/or other gaseous species, such as anultra violet light source, a corona discharge device, or any other meansknown in the art for forming radicals or other active species in a gasstream. The catalyst is positioned so that at least a portion of the gasstream contacts at least a portion of the catalytically active sites onthe surface, and the radicals or other active species are introducedinto the gas stream at a position upstream of the downstream end of thecatalyst. The at least one device for producing radicals or other activespecies from water vapor and/or other gaseous species may be positionedwithin the gas stream, such that radicals are produced directly in thegas stream, or it may be positioned remotely, producing the radicals orother active species either from a portion of the gas stream that hasbeen diverted to the remote device or from some other source of gas.

Preferably, the radicals or other active species are introduced in anamount sufficient to reduce or eliminate poisoning of the catalyst bycatalyst poisons, such as sulfur, sulfur containing compounds,phosphorous, phosphorous containing compounds, or carbon.

Typically, the catalyst is a part of a fuel cell catalytic reactor, anautomotive catalytic convertor, or a catalytic combuster. Where, the gasstream is an exhaust stream from an internal combustion engine. Theinternal combustion engine may be a stoichiometric engine, a lean burnengine, a diesel engine, or any other known type of engine.

For use with an internal combustion engine, the apparatus of theinvention may further comprise a catalytic convertor, having an inletand an outlet, and comprising the at least one catalyst, where thecatalytic convertor is positioned such that at least a portion of theexhaust stream from the engine passes through the catalytic convertor.Typically, an exhaust pipe is attached to the inlet of the catalyticconvertor, such that at least a portion of the exhaust gas stream passesthrough the exhaust pipe to and through the catalytic convertor and theat least one catalyst, and at least one of the catalytic convertor orthe exhaust pipe comprises a fitting for positioning a device forproducing radicals or other active species in the exhaust stream or aportion thereof, so that a radicals or other active species are producedin the exhaust stream upstream of the downstream end of at least onecatalyst in the catalytic convertor. To prevent water that may condenseduring cool down, the at least one device for producing radicals orother active species may be positioned on top of the exhaust pipe orcatalytic convertor.

The at least one device for producing radicals or other active speciesis preferably a corona discharge device, having a power supply.Preferably, the corona discharge device and the power supply are eachdesigned to have a mechanical natural resonant frequency significantlyhigher than that produced by the internal combustion engine. Preferably,the power supply is a low power power supply, typically producing nomore than about 200 watts of power, preferably no more than about 100watts of power, most preferably no more than about 30 watts of power.

The corona discharge device may be positioned such that naturallyoccurring pressure fluctuations in the exhaust stream provide a pumpingaction that forces exhaust gas into the corona discharge device, andscavenges gases containing radicals produced in the corona dischargefrom the corona discharge device. To increase the benefit obtained fromthese pressure fluctuations, a plenum may be positioned adjacent to thecorona discharge device, such that exhaust gas pass from the exhaustpipe, through the corona discharge, into the plenum, and back into theexhaust pipe.

The apparatus of the invention may also comprise a device for injectingair into the exhaust stream during fuel rich cold start operatingconditions, such that the corona discharge causes the combustion ofresidual fuel in the exhaust stream.

The apparatus of the invention may also utilize a remote device forgenerating the radicals and other active species. With an internalcombustion engine, this embodiment further comprises an exhaust pipeattached to the inlet of the catalytic convertor, a tailpipe attached tothe outlet of the catalytic convertor, such that at least a portion ofthe exhaust stream passes from the exhaust pipe to and through thecatalytic convertor and through the tailpipe. The tailpipe has anexhaust gas takeoff for conveying a portion of the exhaust stream to aremote radical generator, which comprises the at least one device forproducing radicals or other active species in the exhaust gas in theportion of the exhaust stream conveyed to the remote radical generator.An output from the remote radical generator returns the exhaust gascontaining radicals or other active species from the remote radicalgenerator to the exhaust stream at a point upstream of the downstreamend of at least one catalyst in the catalytic convertor, where theexhaust gas containing radicals is injected into the exhaust stream.

In a further embodiment, the invention is directed to an apparatus forreducing at least one pollutant in an exhaust gas stream containing anexhaust gas formed from the combustion of fuel in a combustion gasstream, which comprises a precombustion gas stream and the exhaust gasstream. The combustion gas stream may be that of a catalytic combuster,internal or external combustion engine, furnace, boiler, fuel cellcatalytic fuel reformer, electrical power generator, or any other devicethat obtains energy from the combustion of fuel to which a catalyst canbe adapted to reduce the emission of pollution.

The apparatus comprises at least one catalyst, having an upstream endand a downstream end, where the at least one catalyst is positioned suchthat at least a portion of the exhaust gas stream passes through the atleast one catalyst, and at least one device for producing radicals orother active species from water vapor or other gaseous speciespositioned in the combustion gas stream, wherein the radicals areintroduced into the combustion gas stream upstream of the downstream endof the at least one catalyst. Preferably, the device for producingradicals or other active species is a corona discharge device.

In a further embodiment, the apparatus is directed to a fuel cellcatalytic reformer comprising a partial oxidation stage, a catalyticreactor stage, and a preferential oxidation stage, wherein the radicalsor other active species are introduced into at least one of the partialoxidation stage, catalytic reactor stage, or preferential oxidationstage comprises a catalyst. As with the embodiments described above, theat least one device for producing radicals or other active species ispreferably a corona discharge device. The device for producing radicalsor other active species is positioned within the gas stream, or may bepositioned remotely.

The present invention is also directed to a method of enhancing a gasphase chemical reaction in a gas stream. The method comprises contactingthe gas stream with at least one heterogeneous catalyst having anupstream end, a downstream end, and at least one surface having aplurality of catalytically active sites, such that at least a portion ofthe gas stream contacts at least a portion of the catalytically activesites, forming radicals or other active gaseous species either directlyin the gas stream or in a remote generator; and introducing the radicalsor other active gaseous species into the gas stream at a point upstreamof the downstream end of the catalyst. Preferably, the radicals or otheractive species are introduced into the gas stream whenever the gasstream is in contact with the catalyst, i.e., the radicals or otheractive species are introduced starting with the initial use of thecatalyst. This is known as time zero injection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side perspective view of an internal combustion enginehaving a catalytic converter:

FIG. 2 is a schematic of an exhaust system incorporating a remote coronadischarge generator of chemically active species.

FIG. 3 illustrates a corona discharge device mounted in an exhaustshunt.

FIG. 4 illustrates a corona discharge device having concentricelectrodes and a dielectric coated inner electrode.

FIG. 5 illustrates a corona discharge device having concentricelectrodes and a dielectric coated outer electrode.

FIG. 6 illustrates a distant ground corona discharge device.

FIG. 7 illustrates a corona discharge device of the type depicted inFIG. 5 equipped with a flame arrester.

FIG. 8 illustrates a compact corona discharge device.

FIG. 9 illustrates a compact corona discharge device having an extendedskirt.

FIG. 10 illustrates a compact corona discharge device equipped with anorifice for injecting air.

FIG. 11 illustrates a corona discharge device mounted in a manner thattakes advantage of the pumping action of pressure variations in theexhaust gas stream.

FIGS. 12a and 12 b illustrate corona discharge devices mounted inconjunction with or incorporating a plenum that augments the pumpingaction of pressure variations in the exhaust gas stream.

FIG. 13 illustrates the use of a corona discharge device with acatalytic convertor.

FIG. 14 is a schematic of a fuel cell catalytic fuel reformer.

FIG. 15 is a schematic of an example of a power supply circuit for usewith a corona discharge device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein the term “gas stream” refers to any flow of gas to, from,through, or over an article or device. The term “upstream” refers to aposition in the gas stream located relative to a second position in thegas stream in the direction opposite to the flow of the gas stream,i.e., in the direction of the source of the gas stream, and the term“downstream” refers to a position in the gas stream located relative toa second position in the gas stream in the direction of the flow of thegas stream. Therefore, where a point is located upstream of, e.g., acatalyst or a part of a catalyst, the point is positioned in the gasstream between the catalyst or part of the catalyst and the source ofthe gas, and, where a point is located downstream of, e.g., a catalystor a part of a catalyst, the catalyst or part of the catalyst it ispositioned in the gas stream between the source of the gas and thepoint.

As used herein, the term “pre-combustion gas stream” refers to the flowof air or of the air/fuel mixture to the combustion chamber. The terms“postcombustion gas stream” and “exhaust gas stream”, as used herein,refer to the resulting flow of exhaust gases from the combustion chamberfollowing combustion or oxidation of the air/fuel or oxidant/fuelmixture. The pre-combustion and postcombustion gas streams arecollectively referred to as the “combustion gas stream”.

As used herein, the term “catalytic combuster” refers to any device inwhich a fuel is combusted or oxidized on the surface of a heterogeneouscatalyst. That is, any device in which the reaction of a fuel andoxidizer is enhanced by contact with a heterogeneous catalyst.

In addition, the terms “radical” or “radicals” and “free radical” or“free radicals” refer to any atom or group of atoms having at least oneunpaired electron and no net electrical charge; i.e., as used herein,these terms refer to electrically neutral species having equal numbersof electrons and protons, such as hydroxyl radical, OH, and hydrogen andoxygen atoms, H and O respectively, which may also be represented byOH*, H*, and O*, where “*” represents the unpaired electron.

As used herein, the terms “gas phase heterogeneous catalyst” and“heterogeneous catalyst” refer to any non-gaseous catalytic materialhaving a surface that enhances the rate or efficiency of a gas phasereaction. i.e., a chemical reaction that alters the chemical structureof at least one gaseous chemical species.

As used herein the terms “automotive catalytic convertor” and “vehicularcatalytic convertor” refer to any catalytic device that may be used toreduce the emission of pollutants produced by the combustion of fuel inthe engine of an automobile, truck, or motorcycle, or any other type ofvehicle or device that uses an internal or external combustion engine asa source of power.

As used herein, the term “introduction of radicals into the gas stream”includes the introduction of radical and/or related oxidizing speciesthat were produced in a remote radical generator and the direct in situproduction of radicals and/or oxidizing species directly in the gasstream.

The present invention is directed to an apparatus and method forenhancing the rate and efficiency of gas phase reactions within a gasstream and for maintaining and improving the efficiency and performanceof the heterogeneous catalysts used to enhance the rate of such gasphase reactions. Typically, the heterogenous catalysts are of the typefound in catalytic combusters, fuel cell catalytic reformers, andautomotive catalytic convertors.

In the present invention, highly oxidizing free radicals, such ashydroxyl radicals, OH, hydroperoxyl radical, HO₂, atomic hydrogen, H,and atomic oxygen, O, and other active species, including relatedoxidizing gaseous species, such as hydrogen peroxide, H₂O₂, nitrogendioxide, NO₂, and ozone, O₃, are produced in or added to a gas stream,such as, e.g., the combustion gas stream of a catalytic combuster or ofan internal combustion engine equipped with a catalytic convertor, orthe gas stream of a fuel cell catalytic reformer, where the gas streampasses over or through a heterogeneous catalyst in a manner that allowsat least a portion of the gas stream to contact at least a portion ofthe catalyst. The radicals and oxidizing species are produced in eithera remote generator, and then introduced into the gas stream, or, wherethe gas stream contains chemical species that can be converted into thedesired radicals or oxidizing species under the proper conditions, theradicals and oxidizing species may be formed directly within the gasstream. The radicals may be introduced or produced at any point withinthe gas stream upstream of the downstream end of the catalyst, i.e., theportion of the catalyst farthest from the source of the gas stream.

The introduction of radicals into the gas stream results in at least oneof the following:

1. An increase in the rate of the catalytic removal or conversion ofcertain chemical species in the gas stream, including the removal ofpollutants from an exhaust stream by a catalytic convertor, or theconversion of a hydrocarbon fuel to hydrogen gas in a fuel cellcatalytic reactor.

2. The removal of poisons from active sites on the catalyst surface, orthe prevention of the adsorption of catalyst poison onto the catalystsurface, which improves and maintains the efficiency of the catalyst.

3. An increase in the rate and efficiency of oxidation reactions withinthe gas stream before contact with the catalyst.

In addition, maintaining the efficiency of the catalyst improvesreliability, and obviates the need for catalytic overcapacity, therebyreducing volume and weight. Good results have been obtained byintroducing the radicals and/or oxidizing species anywhere upstream ofthe downstream end of the catalyst.

The radicals and related gaseous oxidizing species enhance the oxidationof CO and HC to carbon dioxide (CO₂) and water (H₂O) in a catalyticconvertor or combuster, and, in a fuel cell catalytic reformer, theconversion of fuel to carbon monoxide and gaseous hydrogen (H₂) in afirst step, and then the conversion of the carbon monoxide formed in thefirst step to carbon dioxide. In addition, in internal combustionengines equipped with catalytic convertors, the introduction of radicalsand/or active gaseous species also enhances the reduction of NO_(x) tomolecular nitrogen (N₂).

In particular, it has been observed that hydroxyl radical, OH, can reactrapidly with CO to produce CO₂. It has also been observed that OH in thepresence of oxygen can react rapidly with hydrocarbons (HC) to produceformaldehyde or other similar intermediary products, which then furtherreact with OH to form H₂O and CO₂, and regenerate OH. Therefore, itappears that these reactions do not necessarily consume OH, but,instead, regenerate OH, so that OH acts as a homogeneous catalyst.

In one embodiment, the present invention is directed to a method and anapparatus for the reduction of the amount of pollutants, such as carbonmonoxide (CO), hydrocarbons (HC), and oxides of nitrogen (NO_(x)), inthe exhaust gas stream produced by the high temperature combustion offuel. The method and apparatus of the invention are useful with internalcombustion engines equipped with at least one catalytic convertor in theexhaust system. Preferably the method and apparatus of the invention areused with an internal combustion engine further comprising at least oneoxygen sensor upstream of the catalytic convertor. The oxygen sensorprovides data to the fuel injection system of the engine that allows thefuel injection system to maintain a stoichiometric air/fuel ratio.

It has been discovered that the presence of OH, as well as that of otheractive or reactive species, such as other free radicals and gaseousmolecular intermediates and oxidizers, including O, H, NO₂, H₂O₂, HO₂,and O₃, in the exhaust gases of a combustion engine in the presence ofthe requisite oxygen, provides a highly effective catalytic conversionof CO and hydrocarbons to non-polluting gas species, i.e., CO₂ and watervapor. The OH and other related free radical and gaseous molecularoxidizers created by reaction of OH with gaseous species in the exhauststream act as catalysts independent of or in conjunction with the normalcatalytic function of the catalytic converter.

Thus, the invention employs radicals, such as hydroxyl radical andactive or reactive species, such as O, H, NO₂, H₂O₂, HO₂, and O₃, toprovide a catalytic cycle for reducing CO and HC outputs of engines tomeet present and future Ultra Low Emissions Vehicle “ULEV” and LowEmissions Vehicle “LEV” standards. Because the OH and other freeradicals and active gaseous molecular oxidizing species act ascatalysts, relatively small amounts of radicals need to be injected fororders of magnitude more CO and hydrocarbons to be reduced to CO₂ andH₂O in the presence of oxygen in the exhaust gas stream.

The introduction of radicals and related gaseous oxidizing species intothe combustion gas stream upstream of downstream end of the catalyst ina catalytic convertor results in the catalysis of the oxidation of COand HC in the exhaust gas stream, and provides for the rapid removal ofthose pollutants. The catalytic conversion of CO to CO₂ and hydrocarbonto CO₂ and H₂O by these oxidizing species occurs on the large surface inthe catalytic converter, as well as in the gas phase in the exhauststream. The enhanced conversion of CO and HC to CO₂ and H₂O by radicalsand other active species frees the bulk of the precious metal catalyticsurface from participating in these competing reactions. The converter'sprecious metal sites no longer need to play as strong a role incatalyzing the less reactive hydrocarbon species, such as methane,ethane, ethene, benzene and formaldehyde, and, as a result, thecatalytic activity at the precious metal sites can be directed towardreduction of nitrogen oxides to nitrogen and other non-polluting gasspecies.

Because the catalytic action of the radicals and related gaseousoxidizing species, such as hydroxyl radical, occurs throughout thevolume of the exhaust gas, as well as on the surface of the catalyticconverter, the present invention is significantly more effective than acatalytic converter operating in the conventional manner in reducing theemission of pollutants. The introduction of these radicals for oxidizinggaseous species upstream of the downstream end of the catalyticconvertor also significantly reduces the emission of nitrogen oxidesbelow the level obtained with conventional methods because the preciousmetal sites are freed from the conversion of CO and HC, and, thus, alsoallows a reduction in the amount of precious metals in the catalyticconvertor or the use of less costly metals or their oxides, whilemaintaining the reductions in NO_(x) that are obtained with prior artmethods.

In addition, it has been discovered that the generation of radicals andrelated gaseous oxidizing species, and their introduction into theexhaust stream upstream of the downstream end of the catalyst in acatalytic convertor, cleans the catalytic convertor by reacting with andremoving poisons on the active sites of the surfaces of the catalyticconvertor, as well as preventing the adsorption or deposition ofcatalyst poisons onto the active sites of the catalyst. Catalyst poisonsthat the oxidizing action of these free radicals and related gaseousoxidizing species remove or prevent from being adsorbed include, but arenot limited to, sulfur compounds, such as sulfates and sulfides of thenoble metals in the catalyst, as well as SO and elemental sulfur, whichmay be bound to the surface forming a coating, phosphorous compounds,such as phosphides and phosphates of the noble metals, as well as PO₂,P₂O₃, and elemental phosphorous, which may also be bound to the surfaceof the catalyst forming a coating, and carbon compounds, such as carbonmonoxide, which is adsorbed onto the surface, and can dissociate intoatomic oxygen and carbon, resulting in carbonation.

The oxidation of catalytic poisons from the surfaces of the catalyticconvertor removes the poisons from the catalytic surfaces so that theefficiency of the catalyst is improved, allowing the effective use of acatalyst bed having a smaller volume than that used in a typicalcatalytic convertor today. Therefore the introduction of free radicalsand related gaseous oxidizing species has two independent effects thatreduce the emission of pollutants. First, the catalytic action of theradicals and related gaseous oxidizing species directly removespollutants from the exhaust gas stream. In addition, the removal of allor some of the poisons on the catalyst bed surfaces, in particular, thesurfaces of the noble metals, improves the efficiency of the removal ofpollutants, NO_(x) in particular, by the catalytic convertor.

Referring to FIG. 1, a typical configuration for a modern automobileengine 11 having a catalytic converter 13 is illustrated. The catalyticconverter 13 is positioned at the underbody of the automobile (notshown), and is situated in the exhaust gas stream 18 from the engine, inthe exhaust pipe 12 downstream from the exhaust manifold 15, and beforethe muffler 17. Although this is the configuration commonly used today,it should be noted that a growing number of automobiles are beingproduced with closely coupled catalytic convertors that are positionedcloser to the engine than shown in FIG. 1, such that the catalyticconvertor is adjacent to or part of the exhaust manifold of the engine.In most automobiles produced today, an oxygen sensor 14 is positioned inthe exhaust system upstream of the catalytic convertor 13. Data from theoxygen sensor 14 are used by the electronic controller of the fuelinjection system to maintain a stoichiometric air/fuel ratio. Often, asecond oxygen sensor 16 is located just downstream of the catalyticconvertor to provide additional data for the fuel injection controllerand the on board diagnostics of the vehicle.

The catalytic converter 13, as contemplated for use in the presentinvention, includes any device which is provided for treating exhaustgases from the combustion of a fuel, such as, for example, gasoline,gasoline-based formulations, diesel fuel, alcohol, natural gas and anyother fuel, where a catalytic converter can be used to reduce at leastone pollutant from combustion, such as, for example, CO, HC, and/orNO_(x), including, but not limited to, a three way catalyst typicallyused in today's modern automobile engines.

The catalytic converter 13 therefore comprises any device thatcatalytically removes or participants in the removal of at least onepollutant from an exhaust stream generated by burning a fuel, including,but not limited to, those with monolithic or granular ceramicsubstrates, metallic substrates, or substrates of any kind, and deviceswith noble metals or any other type of catalytic material. It would alsoinclude, without limitation, devices having semiconductor catalysts,such as oxides or sulfides of transition elements, and devices havingceramic-type catalysts, such as alumina, silica-alumina, and zeolitesindividually, in combination with each other and oxygen storage mediasuch as cerium oxide or in combination with metal catalysts.

In one embodiment of the invention, oxidizing radicals and relatedgaseous oxidizing species are introduced into the exhaust streamupstream of the catalytic convertor, 13, and, preferably, upstream ofthe oxygen sensor 14, which is installed in most modern cars and lighttrucks. However, the oxidizing radicals and related gaseous oxidizingspecies may be introduced at any point in the exhaust stream that isupstream of the downstream end of the catalyst of the catalyticconvertor, including the introduction or production of the radicals andrelated gaseous oxidizing species directly into the body of thecatalytic convertor, 13, at a point upstream the downstream end of anyportion of the catalytic convertor that contains catalytic material.Hydroxyl radicals, OH, and atomic hydrogen, H, may be produced fromwater vapor in the exhaust gas of the engine by a radical generatorutilizing any means known in the art for producing radicals, such as UVlight, but, preferably by an electrical corona discharge. Similarly, theradical generator may also produce atomic oxygen, O, from residualoxygen, O₂, in the exhaust gas. Typically, these radical species thenreact with other gaseous species in the exhaust stream to form otheroxidizing species, such as NO₂, H₂O₂, HO₂, and O₃.

The exhaust gas used to produce the free radicals may be taken from thedownstream end of the catalytic convertor by diverting a portion of thedownstream exhaust to a radical generator, and introducing the output ofthe radical generator into the exhaust upstream of the catalyticconvertor, as shown schematically in FIG. 2. By operating the radicalgenerator in exhaust gas taken from the downstream end of the catalyticconvertor, the generator operates in a cleaner environment,substantially free from the pollutants removed by the action of thecatalytic convertor and the oxidizing radicals and active gaseousspecies, which are produced by the discharge, and introduced upstream ofdownstream end of the catalytic convertor. This results in an improveddischarge device lifetime, and substantially eliminates any foulingproblems that may occur when the radical generator is positionedupstream of the catalytic convertor. However, when a corona dischargedevice is used upstream, the corona discharge itself should naturallyreduce or eliminate its own potential contamination.

As shown in FIG. 2, a portion of the cleaned exhaust gas stream 21 thathas passed through the catalytic convertor 13 is taken from the rearexhaust pipe 22, and diverted to the remote radical generator 23. Theoutput 24 of the remote radical generator 23 is enriched with radicalsas a result of the action of, e.g., UV light or a corona discharge onthe exhaust gas 21, and is introduced into the exhaust gases in thetailpipe 12 upstream of the downstream end of the catalytic convertor13. Preferably, an oxygen sensor 14, such as that found on most moderncars and light trucks, is positioned in the exhaust stream 18 upstreamof the catalytic convertor 13, but downstream of the point 25 where theoxidizing species are introduced into the exhaust stream. However,because of the higher pressures in the exhaust system, pumping, such aswith a Venturi (not shown), is required to accomplish direct injectionof the output of a remote generator into the exhaust gas stream.Therefore, the direct, in situ production of free radicals by the actionof a corona discharge on water vapor and residual oxygen in the exhauststream is the most preferred method.

Preferably, the radicals and related gaseous oxidizing species areproduced in the exhaust upstream of the downstream end of the catalyticconvertor by a corona discharge device, placed in either the mainexhaust pipe or in a shunt path in parallel with the main exhaust gasstream, as shown in FIG. 3. As shown in FIG. 3, a corona dischargedevice 30 is mounted in an exhaust shunt 31 in mount 32. The exhaustshunt 31 allows a portion of the exhaust gas stream 18 to bypass asection of the exhaust pipe 12, by exiting the exhaust pipe 12 at afirst point 35, typically upstream of the catalytic convertor 13, andre-entering the exhaust pipe at a second point 36, which is alsoupstream of the catalytic convertor 13. The exhaust shunt may require arestrictive orifice 33 or other device in the exhaust pipe to regulateor control the exhaust gas flow rate. Such a shunt path is useful inthat it allows the corona discharge device to be operated in a lowertemperature environment than that of the exhaust gas stream. Preferably,the heat loss of the shunt path is improved by providing an increasedsurface area with, e.g., cooling fins 34 or similar devices.

A lower temperature environment simplifies the design and choice ofmaterials for the corona discharge device, particularly with regard tothe electrical properties of the device during high temperatureoperation and its thermal design. This is particularly important,because the resistivity, loss tangent, and dielectric constant of thematerials in the corona discharge device change with increasingtemperatures. The change in these properties that occurs at hightemperatures can seriously degrade the efficiency of the coronadischarge device, decreasing the production of free radicals, and, thus,increasing the emission of pollutants. Where a corona discharge deviceis operated in a high temperature environment, the choice of materialsis limited to those that experience a limited change in electricalproperties with increasing temperatures. However, where the coronadischarge device is operated in a lower temperature environment, such asthat of a shunt path, other, less expensive materials that possess thedesired electrical properties at lower temperatures, but lack thedesired properties at high temperature may be used.

Operation at lower temperatures also reduces or eliminates problemsrelated to a mismatch in the thermal coefficient of expansion ofmaterials in the corona discharge device, its support, and the exhaustpipe. This reduces or eliminates strain induced material and sealfailures, as well as failures caused by the numerous thermal cycles thecorona discharge device will experience during the lifetime of theengine.

The free radicals or other active species may also be produced by acorona discharge device 30 mounted within the catalytic convertor 13. Asshown in FIG. 13, a typical three way catalytic convertor comprises anouter steel shell or container, 131, and a plurality, in this case twohoneycomb catalyst “bricks”, 132. The corona discharge device 30, asshown in FIG. 13, may be mounted between the two honeycomb catalyst“bricks”, 132, or at any other position that introduces the radicals ata point upstream of the downstream end of at least one of the twocatalyst bricks, 132. In addition, the radicals may be produced in aremote radical generator, such as that shown in FIG. 2, and thenintroduced into any point in the catalytic convertor upstream of thedownstream end of the catalyst in the catalytic convertor.

The free radicals and other active species may also be produced in thepre-combustion gas stream by a corona discharge upstream of the pointthat the air enters the engine. A drawback of the production orinjection of the oxidizing species in the intake manifold is that asignificant fraction of the highly chemically active species may bedestroyed in the combustion process, and only those active species thatreside in the crevice regions and at the walls of the combustion chambercan effectively survive, and enter into the exhaust gas stream. Incontrast, generators that inject free radical and gaseous molecularoxidizers directly into or which create these species in the exhaust(postcombustion) gas stream can more effectively deliver the activespecies into the exhaust stream where CO and HC need to be oxidized.Thus, the relative amount of radicals that must be produced to provide agiven amount of radicals at the catalytic convertor is significantlysmaller when the active species are produced in or introduced into theexhaust gas stream than the amount required for other methods. Thisdirectly translates into proportionally lower electrical input demandsfor the radical generator.

In a further embodiment, the present invention is directed to a methodand an apparatus for producing gaseous hydrogen from a liquid or gaseoushydrocarbon as fuel for use in a fuel cell. A typical fuel cellcatalytic reformer, 140, for converting hydrocarbon fuel to H₂ for usein a fuel cell is shown schematically in FIG. 14. A liquid or gaseoushydrocarbon fuel, 120, such as gasoline, methane, methanol, or ethanol,is stored in a fuel tank, 121. Fuel, 120, from tank, 121, is vaporized,if necessary, forming a gas stream, and is introduced into a partialoxidation reactor, 123, typically, through a connection pipe , 122,where the fuel is partially burned with a small amount of air to produceH₂ and CO. The partial oxidation of the fuel in the partial oxidationreactor, 123, may be performed in the presence of a catalyst.

The resulting gas stream, comprising a mixture of nitrogen, N₂, CO, andH₂, is then passed into a catalytic reactor, 124, where steam is addedin the presence of a catalyst to remove CO and produce additional H₂ bythe reaction

CO+H₂O→CO₂+H₂.

The remaining CO is preferentially oxidized in the presence of apreferential oxidation catalyst, 125, resulting in a mixture of H₂, CO₂,H₂O, and N₂, which is sent to the fuel cell, 126, where the H₂ iscombined with O₂ to form water and electricity. As NO_(x) is not presentin any of the gas streams of the catalytic reformer, only an oxidationcatalyst is require, and, thus, reduction catalysts, such as rhodium,need not be used.

As with automotive catalytic convertors, it has been discovered that thepresence of OH, as well as that of other free radical and gaseousmolecular intermediates and active species, such as O, H, NO₂, H₂O₂,HO₂, and O₃, in the gas stream of the reformer, provides a highlyeffective catalytic conversion of CO, as well as eliminating orsubstantially reducing the amount of catalyst poisons in the gas streamor on the surface of the catalysts in the reformer. The benefits of theinvention may be obtained by introducing radicals into any gas stream inthe reformer at any point upstream of the downstream end of any of thecatalysts used in the fuel cell catalytic reformer or reformers.

Introduction of radicals into the catalytic reactor stage, 124, servestwo functions. First, hydroxyl radicals can react with CO to form carbondioxide and additional hydrogen gas by the reaction

2CO+2OH→2CO₂+H₂.

In addition, the presence of radicals and other oxidizing species willremove adsorbed catalyst poisons, and prevent the adsorption of catalystpoisons onto the surface of the catalyst, providing for better CO to CO₂conversion. The addition of radicals to the preferential oxidationcatalyst will convert CO to CO₂ and H₂, and remove and prevent theadsorption of poisons in the same manner.

However, because of the high reactivity of the oxidizing radicals andother active gaseous species with hydrocarbon fuels, the radicals may beintroduced into the partial oxidation reactor only when the hydrocarbonfuel is not present. Should the highly reactive radicals be introducedinto the partial oxidation reactor when fuel is present it is likelythat most of the radicals would be consumed by reaction with the fuel,and, thus, the radicals would have little effect on catalyst poisons inthis stage of the catalytic reformer. Therefore, the radicals arepreferably introduced into the partial oxidation reactor only during a“cleanup” or “recovery” cycle in which any catalyst poisons adsorbedonto a catalyst in the catalytic reformer, in particular, any catalystused in the partial oxidation reactor, may be oxidized and removed toclean and recover the catalyst. If a “cleanup” or “recovery” cycle ormode is required, it may be desirable to provide a pair of catalyticreformers in parallel for a fuel cell that will be used on asubstantially continuous basis. In this manner, one catalytic reformermay be used to provide hydrogen fuel for the fuel cell, while the secondreformer is in the recovery mode. By cycling between reformers in thismanner, each reformer could always be operated with a minimum ofcatalyst poison adsorbed onto the catalyst.

However, it has also been discovered that the generation of radicals andrelated active gaseous species, and their introduction into a gas streamin the catalytic reformer upstream of the downstream end of a catalystin either the catalytic reactor or the preferential oxidation catalystduring operation of the catalytic reformer, cleans the catalyst in thosestages by reacting with and removing poisons on the active sites of thesurfaces of the catalytic convertor, as well as preventing theadsorption or deposition of catalyst poisons onto the active sites ofthe catalyst.

As with automotive applications of the invention, hydroxyl radicals, OH,and atomic hydrogen, H, are produced from water vapor in a gas stream ofthe catalytic reformer. The radicals may be formed by a radicalgenerator utilizing any means known in the art for producing radicals,such as UV light, but, preferably by an electrical corona discharge.Similarly, the radical generator may also produce atomic oxygen, O, fromresidual oxygen, O₂, in the exhaust gas. Typically, these radicalspecies then react with other gaseous species in the exhaust stream toform other oxidizing species, such as NO₂, H₂O₂, HO₂, and O₃. Where theradicals are produced during the operation of the catalytic reformer, itis preferred that the radicals be produced in at least one of thecatalytic reactor, the preferential oxidation reactor, or within a gasstream supplying air or water vapor to at least one of the catalyticreactor or the preferential oxidation reactor. As with the automotiveapplications, it is preferred that the radicals be produced with acorona discharge device of the type described below.

A corona discharge device for use with the invention should preferablybe capable of functioning for at least about 3,000 to about 4,000 hoursin the high temperature environment of the exhaust stream of an internalcombustion engine before replacement is required. Because of spacelimitations in modern automobiles and in applications using fuel cells,it is preferred that the corona discharge device have a small physicalvolume, i.e., on the order of the size of a typical spark plug, andrequire a power supply that is no larger than about 300 to about 400cubic cm. In certain embodiments, in addition to operating at atemperature on the order of about 800° C., the corona discharge devicemust meet automotive electromagnetic interference (EMI) requirements, bereadily replaceable, and be capable of withstanding thousands of thermaltransients of about 800° C., such as those experienced during start-upand cool down of an engine, as well as several million smaller thermaltransients where the change in temperature may be on the order of about200° C. In a preferred corona discharge device, about 20 to about 50 Wof high frequency, high voltage power is required, i.e., from about1,000 to about 250,000 Hz and from about 5,000 to about 20,000 VAC.However, under some transient operating conditions, such as engine coldor warm starts, more radical production may be desired. In this case thecorona device would require operation at higher power levels of up to200 to 300 watts. This transient power condition can be met byincreasing the frequency voltage product to the corona device by afactor of 5 to 10 for such periods, which typically range from about 30to about 100 sec. This can be accomplished through proper corona unithigh voltage power system design, and the use of control signals fromthe engine controller or local startup temperature readings.

An example of the circuitry for a power supply useful with the presentinvention is shown in FIG. 15. The circuitry shown in FIG. 15 provides aresonant switch-mode invertor capable of converting a 12 VDC nominalinput voltage to an approximately 10 kVAC sine wave output to drive acapacitive “silent discharge” device, such as the corona dischargedevice of the invention.

Corona discharge devices useful in the invention include, but are notlimited to, those having generally cylindrical symmetry and, in mostcases, at least two concentric electrodes. At least three general designalternatives for corona discharge devices that have generallycylindrical symmetry exist. These three general design alternatives areillustrated in FIGS. 4, 5, and 6. FIG. 4 is a cross-section of acylindrical corona discharge device 40 having concentric cylindricalelectrodes inner electrode 41 and outer electrode 42. The device 40typically includes a ferrule 44 in the base 47, which provides a gasseal, and threads 46 or other means for mounting the device 40 in theexhaust pipe 12 or shunt 31. The inner electrode 41 is surrounded by adielectric layer 43, which prevents breakdown, and maintains the coronadischarge. It is important for the overall efficiency of the device tohave the predominant voltage across the “air” gap 45 of the device.Because the dielectric layer 43 in the corona discharge device shown inFIG. 4 is located in a region where high electric fields occur, most ofthe voltage is across the “air” gap of the corona discharge device, andthe efficiency of the device is maintained.

However, depending on the design of the corona discharge device, thedielectric, due to its conductivity, may act as a shunt conductive pathto ground that effectively reduces the current to the corona discharge.Where the corona discharge device is subject to shunt capacitive lossesin the region of the base 47 that increase proportionally withincreasing dielectric constant, a decision is often required during thedesign of a corona discharge device of this type, as to the relativeimportance of the voltage drop across the dielectric and the shuntcapacitive losses in the base region. In practice, the careful design ofthe corona discharge device will minimize the effective area of theshunt capacitance, and provide a low dielectric constant.

Resistive losses also occur in dielectrics at high temperatures, and,thus, a dielectric material must be selected in which the resistivelosses are acceptably low, or the corona discharge device must beoperated in a chamber or shunt path off of the exhaust system to allowoperation at a lower temperature. Other design issues include EMI,resistance to corrosion in the corrosive, high temperature environment,contamination, condensation of water during engine cool down, andvibration. For EMI, the corona discharge device and its power supply andleads must have sufficient shielding to meet automotive system EMIrequirements.

Material selection should be based on high temperature behavior and theability to withstand a corrosive environment that could limit the designlife or performance of the device, e.g., high temperature diffusion ofcontaminants into the dielectric that could lower the resistivity of thedielectric below the required value for maximum efficiency, and possiblyresult in the formation of a partial or complete short circuit in thedevice. However, the corona discharge itself should naturally reduce oreliminate contamination of the device.

The need for a high dielectric constant can be reduced or eliminated byplacing the dielectric layer 43 on the inner surface of the outerelectrode 42. Such a device 50 is illustrated in FIG. 5. Because theelectric fields that occur in the region of the outer electrode 42 arerelatively low compared to those in the region of the center electrode41, a dielectric material having a lower dielectric constant may be usedfor the dielectric layer. This reduces shunt capacitive losses, whilemaintaining a limited voltage drop across the dielectric layer.

It is also possible to use, for example, the exhaust pipe 12 or exhaustshunt 31 as a distant ground for the corona discharge device,eliminating the need for an outer electrode. Such a distant groundcorona discharge device 60 is shown in FIG. 6, and only requires aninner electrode 41, preferably, with a sharp or small radius tip topromote breakdown, a dielectric insulator 43, and a base 47, whichtypically includes a ferrule 44 to provide the required seal and strainrelief. Because a distant ground device is only subject to base lossconsiderations, such a device also allows the use of dielectricmaterials having a low dielectric constant.

It may also be desirable in some applications to include one or moreflame arresters in the design of the corona discharge device. Such adevice is shown in FIG. 7, in which a corona discharge device 50 havingan outer electrode 42 coated with a dielectric layer 43 is capped with aflame arrester 48 in the form of a wire screen. Such a flame arresterwill prevent the ignition of exhaust gases containing fuel and oxygenduring engine starts and misfires.

However, in some internal or external combustion engine applications,the ignition of exhaust gases to initiate partial or complete combustionof residual fuel in the exhaust gases is desirable, thereby reducingharmful emissions, such as, e.g., during the cold start phase of theengine operation or under conditions where the engine misfires. Suchcorona assisted combustion of residual fuel and hydrocarbons is possiblewithout the production of additional NO_(x) due to the low temperatureof the combustion process in the exhaust stream.

Under conditions where the engine misfires, the fuel air mixture will besubstantially stoichiometric, and no additional air is required toinitiate combustion of the resulting exhaust gas. However, to initiatecombustion of the residual fuel in the exhaust during cold startconditions, additional air must be added to the exhaust gas streamupstream of the corona discharge device, as the exhaust gases are fuelrich under those conditions. The oxygen required for combustion can beprovided through controlled injection of air, either by self pumping,such as through the pumping action of a Venturi section in the exhaustpipe, or by an upstream air pump. With a Venturi, a fast acting valve,such as an electro-mechanical valve or a valve based on MEMS (MicroMechanical-Electronic Systems) technology would be required to terminatethe air injection after the cold start period was complete. The rate ofair injection is limited with a Venturi, and, thus, only partialcombustion of residual fuel is possible with Venturi pumping. However,an air pump is not subject to such a limitation, and can providesufficient air for complete combustion of any residual fuel in theexhaust gas stream.

Where the ignition of exhaust gases by the corona discharge is desired,it may also be desirable to use flame arresters, such as wire screen tocontrol or limit the regions of the exhaust stream in which coronaassisted combustion could occur to any of, e.g., upstream of the coronadischarge device, downstream of the device, both upstream and downstreamof the device, or in a limited volume in and around the corona dischargedevice.

Corona discharge devices useful in the present invention may be of anytype that produces a corona sufficient to form an effective amount ofactive chemical species, such as hydroxyl radical. For example,representative corona discharge devices, such as those shown in FIG. 4and FIG. 5, may be modified sparkplug-like devices, having a smallcenter electrode 41 with a diameter of about 0.1 to about 0.3 cm. Theinner electrode 41, is inserted into and held in place by a hole in thedielectric layer 43 in the base 47. In devices where the dielectriclayer 43 is positioned on the inner surface of the outer electrode 42,the dielectric layer 43 basically forms a cup having a hole in its baseto position the inner electrode. The outer electrode has an innerdiameter of about 1 to about 2 cm and a length of about 1.5 to 3 cm. Thedielectric layer has a base and wall thickness of about 1 to about 3 mm,which is chosen to provide the desired dielectric strength at theoperating voltage of the corona discharge device.

The dielectric layer adjacent to the interior wall of the outerelectrode and the “air gap” between the dielectric layer and the innerelectrode are essentially two series capacitances. Because they are inseries, the currents through the air gap and the dielectric in thisregion are equal, and, thus, the instantaneous corona power dissipationfor cylindrical electrodes may be expressed as

P _(i) =V _(i) I _(d) =ωC _(d) ·V _(s)cos (ωt).

The average power dissipation is then expressed as$P = {{\langle{Pi}\rangle} = {4\quad {C_{d} \cdot V_{s}}{f\left\lbrack {V_{o} - {\left( \frac{C_{d} + C_{g}}{C_{d}} \right)V_{s}}} \right\rbrack}}}$

where C_(d) is the solid dielectric capacitance, C_(g) is the air gapcapacitance, V_(s) is the spark breakdown potential, V_(o) is theapplied voltage, and f=ω/2π.

This means that, using “spark plug” technology, a very compact,replaceable corona discharge unit can be produced, having the requiredpower level.

The outer surface of the outer electrode can be used to mount the coronadischarge device in the exhaust pipe or manifold, an exhaust shunt path,in an anterior chamber to the exhaust pipe, a mounting plate on or inone of these devices, or any other simple means of mounting the coronadischarge device that provides a good exhaust gas seal. This simplemounting scheme allows easy removal and installation of the coronadischarge device in the exhaust system, and with a shunt path or slightrecess in the exhaust system represents little or no interference to themain exhaust flow. In each case, the corona discharge device is placedin the exhaust gas of the engine, so that the desired free radicals areproduced directly from water vapor, residual oxygen, and otherconstituents of the exhaust gas.

The condensation of water during cool down could result in a short outof the corona discharge device, and, thus, the device is preferablymounted in the top of the exhaust pipe, so that the electrodes facedown, minimizing the exposure to water during those times when thetemperature is too low to drive off any water. In addition, vibrationproblems may be avoided by designing the device and its power supply andwiring to have natural resonant frequencies well above automobilevibrational frequencies.

As discussed above, the resistive and capacitive shunt losses of thedielectric layer used to provide an insulating support between the twoelectrodes of a corona discharge device are a major consideration in thedesign of such a device. Any reduction in shunt capacitance allowsoperation of the discharge at higher frequencies at a given capacitivepower loss, and, according to basic design principles for a coronadevice having a power output proportional to the frequency of theapplied voltage, would allow a more compact design. A more compactdesign is advantageous in that it allows the use of a smaller coronagap, which, in turn, results in a lower breakdown voltage across thegap, and, thus, allows the use of a lower operating voltage. The loweroperating voltage results in lower resistive and capacitive losses,increasing the efficiency of the corona discharge device. The smaller,more efficient corona discharge device will thus require a smaller powersupply, which is a major advantage in modern vehicles where space is ata premium.

A representative design for such a compact corona discharge device isshown in FIG. 8. FIG. 8 illustrates the physical components of anefficient compact corona discharge device 80, as well as the importantdevice operating and device design regions. The illustration, as well asthe dimensions given below, is merely representative of a genericdesign, and one of ordinary skill in the art will recognize that manyvariants that fall within the scope of the general design principlesillustrated and discussed here.

The key features of the embodiment illustrated in FIG. 8 include a long,thin-walled dielectric insulator 81 that, along with the properselection of materials, provides a path of high resistance between theinner 82 and outer 83 electrodes that are supported by the insulator 81.A thin metal cap 84 is provided as a gas seal. The inner electrode 82may be substantially longer than the outer electrode 83. In one suchembodiment, the inner electrode 82 typically has a length that is atleast about twice that of the outer electrode 83, and, preferably, atleast about 4 times the length of the outer electrode 83, and the lengthof the inner electrode 82 is typically about at least about 4 times,preferably at least about 6 times, the diameter of the corona dischargedevice 80, as determined from the diameter of the dielectric insulator81. The outer electrode 83 is mechanically and electrically connected tothe base 85 of the compact corona discharge device 80, where the baseincludes threads 86 or other similar mounting means to mount the device80, such that exhaust gases may enter into the air gap 89. As a resultof the difference in the length of the inner and outer electrodes 82 and83, the air gap 89 is divided into a corona discharge region 87, i.e.,that part of the air gap 89 where the inner and outer electrodesoverlap, and a ullage volume 88, i.e., that portion of the air gap 89that extends from the outer electrode 83 to the metal cap 84.

A typical compact discharge device 80 may have an outer electrode 83with a length of about 1 to about 2 cm, preferably about 1.5 cm, and aninner electrode 82 with a length of about 4 to about 8 cm, preferablyabout 5 to about 7 cm, most preferably about 6 cm. The dielectricinsulator 81 of such a device can be constructed from a ceramic materialsuch as Fosterite, and will have a diameter of about 0.7 to about 1.3cm, preferably about 1 cm, a length of about 3 to about 5 cm, preferablyabout 4 cm, and a thickness of about 0.1 to about 0.2 cm, preferablyabout 0.15 cm, can be used at a temperature of up to about 900° C. withresistive power losses of less than about 10% at a maximum operatingvoltage of at least about 5,000 V. In a relatively low temperatureenvironment, such as that in an exhaust shunt, and because of thetemperature variation along the ceramic dielectric insulator 81, an evenhigher operating voltage is possible, while maintaining an acceptablepower loss. A corona discharge device of this design would provide about30 to about 50 W of power operating at a frequency of about 100 kHz.However, under some transient operating conditions, such as engine coldor warm starts, more radical production may be desired. In this case,the corona device would require operation at higher power levels of upto 200 to 300 watts. This transient power condition can be met byincreasing the frequency voltage product to the corona device by afactor of 5 to 10 for such periods, which typically range from about 30to 100 sec. This can be accomplished through proper corona unit highvoltage power system design and the use of control signals from theengine controller or local startup temperature readings. The longinsulating path and thin walls of the insulator 81, minimize thecapacitive shunt losses to less than about 10%, even for insulatorshaving a dielectric constant of more than 10 at operating frequencies onthe order of about 100 kHz. Such a high operating frequency allows theuse of a very compact high voltage power supply.

As discussed above, the expression for the power dissipation in a coronais given by

P=4C _(d) ·V _(s) ·f{V _(o)−((C _(d) +C _(g))/C _(d))·V _(s)},

where C_(d) and C_(g) are respectively the capacitance of the dielectricand the gap in the corona region, V_(s) and V_(o) are respectively thespark breakdown voltage of the gap and the applied voltage to the coronadevice, and f is the frequency of the voltage applied to the device.Taking values of these quantities of as V_(o)=5,000 V, V_(s)=3,000 V,C_(d)=6×10⁻¹² farad, C_(g)=1×10⁻¹² farad; then at a frequency of 3×10⁵Hz, the power in the corona is about 27 W. The output can be scaled byfrequency, applied voltage, or capacitance (primarily the length of thecorona discharge region). The output can be controlled by the frequencyand/or voltage of the corona device power source.

The spark breakdown voltage is almost directly proportional to thedensity of the exhaust gas in the corona gap region, which is almostdirectly proportional to the temperature in the gap region. Thisbreakdown voltage will vary in proportion to the temperature of the gasin the corona unit, and, therefore, its operating temperature. If, forexample, the design were such that the gas temperature in the coronaunit were half of the exhaust temperature, then the lower breakdownvoltage would increase to 6,000 v.

FIGS. 9 and 10 show two design variants on the above design. In FIG. 9the skirt section 91 is lengthened and extended surfaces 92 are employedto augment heat exchange to the ambient environment. The longerconduction path along with the heat exchangers provide for cooleroperation of the dielectric material 81 in particular, thus providingfor a wider selection of materials or better performance for thisapplication with satisfactory resistance and capacitance at theresulting operating temperature. It has also been observed in tests thatthe injection of small amounts of air (<10 cc/sec) in a manner thatmodifies the engine stoichiometry to permit the operation of thecatalyst at a desired equivalence ration, e.g., by injecting airupstream of the engine side oxygen sensor, results in no adverse engineperformance or engine/catalyst emission performance. In FIG. 10 apumping action is provided by the low pressure produced in a Venturisection 95 added to the exhaust system 96. This low pressure inconjunction with the orifice 97 in the metal cap 84 of the compactcorona discharge device 80 provides for an air flow of less than about10 cc/sec, which limits the temperature, cooling the ceramic dielectricsection of the corona device, and aids in the injection of radicalsgenerated in the corona discharge.

Under normal operating conditions, the engine produces exhaust gaspressure oscillations having a frequency of about 30 to about 100 Hz anda peak to peak variation of about 20 to about 80%, depending upon thelocation in the exhaust system. These pressure oscillations inconjunction with the ullage volume 88 provide an effective, continuouspumping action of the radicals and other species produced in the coronadischarge into the exhaust stream. The pumping effect of the exhaust gaspumping oscillations for any of the corona discharge devices describedabove, where the discharge device 110 is installed at a point on theexhaust pipe 112 where the oscillations occur, in the manner shown inFIG. 11, where the discharge device 110 is mounted on a simple “T” 113off the side of the exhaust pipe 112. The pumping effect and the totalgas motion can be augmented with a plenum 114 as shown in FIGS. 12a and12 b. As shown in FIG. 12a, the plenum 114 may be a separate extensionof the exhaust pipe that is adjacent to the corona discharge device.Alternatively, the plenum 114 may be incorporated into the coronadischarge device, as shown in FIG. 12b, such that high pressureoscillations in the exhaust force a portion of the exhaust gas past thecorona discharge into the plenum, and low pressure oscillations in theexhaust force exhaust result in the exhaust gas in the plenum returningto the main exhaust gas stream enriched with radicals and other activespecies. In addition, cooling fins 116 may be added to lower theoperating temperature for the discharge device 110. As noted above, acooler operating environment improves the efficiency of the coronadischarge.

It should be noted that the only requirement of the precedingembodiments of the present invention is that free radicals or gaseousand active oxidizing species, in particular, hydroxyl radical, are addedto the combustion gas stream at a point upstream of or at the catalyticconverter, for example, the air intake duct to the carburetor orfuel-injection systems of the combustion chamber, the air/fuel intakemanifold to the combustion chamber, the combustion chamber directly orthe exhaust manifold of the combustion chamber, or the exhaust pipe.

Moreover, while the present invention has been described in oneembodiment with reference to a catalytic converter, it is contemplatedthat only the high surface area provided by those catalysts inconjunction with the introduction of hydroxyl radicals and other activespecies would be required to reduce the pollutants in the exhaust gasesof a combustion engine.

Although the present invention has been described with particularreference to its preferred embodiments, it should be understood thatmany variations and modifications will now be obvious to those skilledin that art, and, therefore, the scope of the invention should not belimited by the specific disclosure herein, but only by the appendedclaims.

What is claimed:
 1. An apparatus for enhancing the rate of a chemicalreaction in a gas stream of a mobile vehicle, the apparatus comprising:a passageway for channeling the gas stream; at least one device forproducing radicals or other active species from at least one of watervapor or other gaseous species present in the gas stream located in thepassageway; a power supply including a switch-mode inverter capable ofconverting DC input into AC output, the power supply adapted andconfigured to provide electrical power having a frequency of at leastabout 1,000 Hz to the at least one device; and at least one heterogenouscatalyst having an upstream end and a downstream end, and at least onesurface having a plurality of catalytically active sites on the surfacepositioned in the passageway such that at least a portion of theheterogenous catalyst is located downstream from the at least one deviceand at least a portion of the gas stream is exposed to the at least onedevice prior to the downstream end of the heterogenous catalyst.
 2. Theapparatus according to claim 1, further comprising at least one sensorfor monitoring the gas stream located downstream from the downstream endof the heterogenous catalyst.
 3. The apparatus according to claim 2,wherein the power supply is adapted and configured to provide electricalpower of variable current and voltage.
 4. The apparatus according toclaim 3, further comprising a feedback loop connecting the at least onesensor to the vehicle's onboard diagnostics.
 5. The apparatus accordingto claim 1, wherein the radicals or other active species are introducedin an amount sufficient to reduce or eliminate poisoning of the catalystby catalyst poisons.
 6. The apparatus according to claim 5, wherein thecatalyst poison is at least one of the group consisting of sulfur, asulfur containing compound, phosphorous, a phosphorous containingcompound, carbon and a carbon containing compound.
 7. The apparatusaccording to claim 1, wherein the gas stream is an exhaust stream froman internal combustion engine.
 8. The apparatus according to claim 7,wherein the internal combustion engine is a spark ignition engine. 9.The apparatus according to claim 7, wherein the internal combustionengine is a diesel engine.
 10. The apparatus according to claim 1,wherein the at least one device, comprises; at least one firstelectrode; at least one second electrode; and at least one dielectricpositioned between the first and second electrodes.
 11. The apparatusaccording to claim 10, wherein the at least one second electrode isconcentric about the at least one first electrode.
 12. The apparatusaccording to claim 10, wherein the dielectric is a layer on either thefirst or second electrode.
 13. The apparatus according to claim 1adapted and configured such that no additional reducing agent is addedto the gas stream.
 14. The apparatus according to claim 1, wherein theat least one device and the at least one heterogenous catalyst areseparated by a pre-determined distance in the passageway.
 15. Theapparatus according to claim 1, wherein the heterogenous catalyst is athree-way catalyst.
 16. The apparatus according to claim 1, wherein theheterogenous catalyst is a ceramic or zeolite catalyst.
 17. Theapparatus according to claim 1, further comprising a particulate traplocated in the passageway.
 18. The apparatus according to claim 17,wherein the device is continuously powered whenever the internalcombustion engine is operating.
 19. An apparatus for enhancing the rateof a chemical reaction in an exhaust gas stream containing exhaust gasesof a mobile vehicle, the apparatus comprising: a passageway forchanneling the gas stream; at least one device for producing radicals orother active species from at least one of water vapor or other gaseousspecies present in the gas stream located in the passageway, wherein thedevice is adapted and configured to produce the radicals or other activespecies substantially whenever the mobile vehicle is producing theexhaust gases; a power supply including a switch-mode inverter capableof converting DC input into AC output, the power supply adapted andconfigured to provide electrical power having a frequency of at leastabout 1,000 Hz to the at least one device; and at least one heterogenouscatalyst having an upstream end and a downstream end, and at least onesurface having a plurality of catalytically active sites on the surfacepositioned in the passageway such that at least a portion of theheterogenous catalyst is located downstream from the at least one deviceand at least a portion of the gas stream is exposed to the at least onedevice prior to the downstream end of the heterogenous catalyst, whereinthe apparatus is adapted and configured such that no additional reducingagent is added to the exhaust gas stream.
 20. An apparatus for enhancingthe rate of a chemical reaction in an exhaust gas stream containingexhaust gases of a mobile vehicle, the apparatus comprising: apassageway for channeling the gas stream; at least one device forproducing radicals or other active species from at least one of watervapor or other gaseous species present in the gas stream located in thepassageway, wherein the device is adapted and configured to produce theradicals or other active species substantially whenever the mobilevehicle is producing the exhaust gases; a power supply including aswitch-mode inverter capable of converting DC input into AC output, thepower supply adapted and configured to provide electrical power having afrequency of at least about 1,000 Hz to the at least one device; atleast one heterogenous catalyst having an upstream end and a downstreamend, and at least one surface having a plurality of catalytically activesites on the surface positioned in the passageway such that at least aportion of the heterogenous catalyst is located downstream from the atleast one device and at least a portion of the gas stream is exposed tothe at least one device prior to the downstream end of the heterogenouscatalyst; and at least one sensor for monitoring the exhaust gas streamlocated downstream from the downstream end of the heterogenous catalyst,wherein the apparatus is adapted and configured such that no additionalreducing agent is added to the exhaust gas stream.